1. Field of the Invention
The present invention relates, in general, to systems for actively monitoring and/or controlling energy cells in a multiple energy storage cell pack, and, in particular, to systems for actively monitoring and/or controlling multiple interconnected cells of individual capacitors in an ultracapacitor energy storage cell pack.
2. Related Art
An ultracapacitor energy storage cell pack includes multiple interconnected cells of individual capacitors that store an electrical charge. Manufacturing tolerances and variables of an ultracapacitor energy storage cell pack cause differing voltages across the individual cells within the pack during charging and discharging operation. Exceeding the maximum recommended voltage across an individual cell when charging can damage or shorten the life of that cell, which can degrade the performance of the entire pack. Maintaining a working voltage level on each individual cell that is uniform throughout the pack of cells minimizes the stress and prolongs the life of each cell, causing the ultracapacitor energy storage cell pack to have a longer storage life and useful life.
Prior methods to limit the voltage of individual cells have offered either no protection or completely drained the cells. For example, a passive method has been used to limit the voltage of individual cells. The passive method includes using a resistor between positive and negative terminals of the cell. A problem with this method is that it causes a constant energy drain of the cell and results in a complete discharge of the cell over a period of time. Another standard clamping method combines a resistor and a voltage reference diode across the positive and negative terminals of the cell. A problem with this method is that the maximum power dissipation of the diode prevents the proper amount of over-voltage protection.
The performance of an ultracapacitor energy storage cell pack degrades when an individual cell fails to hold a charge. It is cost effective to replace the failed cell rather than discard the whole pack. A prior method for detecting failed cells involved monitoring the whole pack for an out-of-tolerance condition and, if an out-of-tolerance condition exists, sequentially checking each individual cell to locate the failure. This method is undesirable because the time it takes to determine which cells failed in a pack is unpredictable and can be lengthy.